The synthesis and biological activity of a photochromic compound, in which two Lys2 dipeptides are linked through their N-terminal amino groups by an azobenzene moiety, are reported. The synthesis was achieved by a novel solid-phase dimerization strategy, based on the reaction of 4,4prime-azobenzene dibenzoyl chloride with two N-terminal amino groups of distinct Lys2 dipeptides directly on the resin. Both the trans form and the photostationary state mixture (59% cis and 41% trans) inhibit a plant proteinase which is known to be sensitive to polycationic compounds.